I'm making a paella but don't have paella Rice! Could I use orzo?
It's not a direct substitution, but you can use orzo in place of paella rice as long as you vary the liquid. When I make a paella I use 3x the liquid as the rice by volume. With orzo that may be too much - if it doesn't absorb it you'll have a watery paella.
My approach would be to add 1.5 times liquid to the pasta by volume (not including your tomatoes) and see how it goes. If it starts to dry out and the pasta isn't done you can add small amounts of liquid until it is cooked.
FYI, any short grain rice will do for paella, you don't have to buy rice specifically saying paella rice.
I make paella often and any short grain rice is preferred to orzo. If you can find Arborio rice (which is what I use living in middle america rather than the more classical bomba or calasparra rices), that works great in a pinch. You could probably make orzo work if you pay attention to the water content and make sure to cook all the water off by the end, without over cooking the orzo.
Rice is a grain, orzo is a pasta. A small, rice-sized and -shaped pasta, but still a pasta. If you try to substitute, you will have no idea of how much water to use, and chances are you will end up with hideously overcooked orzo. In addition, you cannot expect the starch releasing effect of a paella/risotto rice with orzo, which would simply disintegrate, if you try too hard to stir it.
I would recommend any short-grain rice instead. You can usually find perfectly adequate versions in any Asian grocery store, much cheaper than in more specialized stores. Or just look on the lower shelves of your supermarket, if you're in the US.